fix: trigger onConnected callback on subscribe if already connected

[rolznz]

Fixes #176

Also deprecates isConnected api function

[bumi]

what states do we have or might be interesting:

• isConnected meant that webln is ready and enabled?
• onConnect is called once enable() is done
• do we have something to let the app know that a user has done the setup already before (no matter if enable() is finished or not)

[rolznz]

what states do we have or might be interesting:

• isConnected meant that webln is ready and enabled?
• onConnect is called once enable() is done
• do we have something to let the app know that a user has done the setup already before (no matter if enable() is finished or not)

• isConnected meant that webln is ready and enabled? - yes, but I decided to deprecate that because it is unreliable based on timing
• onConnect is called once enable() is done - yes, or when onConnect is called and the enable was already done before onConnect was called
• do we have something to let the app know that a user has done the setup already before - not yet